1. Field of the Invention
The present invention concerns devices for heating foods on meal trays, in particular for distribution of meals in collectivities, company canteens, hospitals.
2. Description of the Prior Art
Devices of this kind for heating foods are described, for example, in document DE-A-27 11 088, comprising means for supporting a series of trays one above the other with an appropriate distance between them and a series of induction shelves containing induction coils connected to a high-frequency current generator to generate a high-frequency alternating magnetic field, the induction shelves being inserted between the successive trays in a heating area of the trays.
The foods to be heated are placed on plastics materials plates the top surface of which is covered with a thin aluminum bowl, the plates being covered by a removable insulative lid the bottom face of which is covered by a metal film.
The induction coils carrying a high-frequency electric field generate a high-frequency alternating magnetic field in the tray heating area which heats by induction the bottom metal layer covering the plate and the removable top metal layer covering the inside of the lid.
In an induction heating device of this kind the system comprising the induction coils and the top and bottom metal layers carried by the trays form a magnetic circuit the impedance of which differs according to whether trays are present near the coils or some trays are absent or do not carry any metal part. This difference in the impedance modifies the operation of the high-frequency electrical current generator that energizes all of the coils with the result that it is not possible to obtain optimal operation in all cases, i.e. with all the trays present and with at least some of the trays absent. This also causes variation in the rate of heating of the foods, depending on the number of trays present in the series of trays.
Furthermore, with some trays absent, the induction coils generate an interference magnetic field that propagates into the environment of the device. Thus a device of this kind causes electromagnetic interference.
The problem to which the present invention is addressed is that of designing a new method and a device for automatically preventing variation in the rate of heating of the foods when at least some of the trays of the series of trays to be heated are absent.
At the same time, the aim is to avoid the generation of electromagnetic interference, in particular when some trays are absent.
A subsidiary aim is to automate the heating of some trays and the absence of heating of other trays when the user requires this.
Finally, the device may be adapted to produce a lower temperature in a cold part of the tray on which are placed foods that must be served at a lower consumption temperature.